This article describes how to initialize a list with any size [number of elements] and values in Python.
- Create an empty list
- Initialize a list with any size and values
- Notes on initializing a 2D list [list of lists]
- For tuples and arrays
See the following article about the initialization of NumPy array ndarray.
- NumPy: Create an ndarray with all elements initialized with the same value
Create an empty list
An empty list is created as follows. You can get the number of elements of a list with the built-in function len[].
l_empty = []
print[l_empty]
# []
print[len[l_empty]]
# 0
You can add an element by append[] or remove it by remove[].
l_empty.append[100]
l_empty.append[200]
print[l_empty]
# [100, 200]
l_empty.remove[100]
print[l_empty]
# [200]
See the following articles for details on adding and removing elements from lists,
- Add an item to a list in Python [append, extend, insert]
- Remove an item from a list in Python [clear, pop, remove, del]
Initialize a list with any size and values
As mentioned above, in Python, you can easily add and remove elements from a list, so in most cases, it is not necessary to initialize the list in advance.
If you want to initialize a list of any number of elements where all elements are filled with any values, you can use the * operator as follows.
l = [0] * 10
print[l]
# [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
print[len[l]]
# 10
A list is generated that repeats the elements of the original list.
print[[0, 1, 2] * 3]
# [0, 1, 2, 0, 1, 2, 0, 1, 2]
You can generate a list of sequential numbers with range[].
- How to use range[] in Python
Notes on initializing a 2D list [list of lists]
Be careful when initializing a list of lists.
The following code is no good.
l_2d_ng = [[0] * 4] * 3
print[l_2d_ng]
# [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
If you update one list, all the lists will be changed.
l_2d_ng[0][0] = 5
print[l_2d_ng]
# [[5, 0, 0, 0], [5, 0, 0, 0], [5, 0, 0, 0]]
l_2d_ng[0].append[100]
print[l_2d_ng]
# [[5, 0, 0, 0, 100], [5, 0, 0, 0, 100], [5, 0, 0, 0, 100]]
This is because the inner lists are all the same object.
print[id[l_2d_ng[0]] == id[l_2d_ng[1]] == id[l_2d_ng[2]]]
# True
You can write as follows using list comprehensions.
- List comprehensions in Python
l_2d_ok = [[0] * 4 for i in range[3]]
print[l_2d_ok]
# [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
Each inner list is treated as a different object.
l_2d_ok[0][0] = 100
print[l_2d_ok]
# [[100, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
print[id[l_2d_ok[0]] == id[l_2d_ok[1]] == id[l_2d_ok[2]]]
# False
Although range[] is used in the above example, any iterable of the desired size is acceptable.
l_2d_ok_2 = [[0] * 4 for i in [1] * 3]
print[l_2d_ok_2]
# [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
l_2d_ok_2[0][0] = 100
print[l_2d_ok_2]
# [[100, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
print[id[l_2d_ok_2[0]] == id[l_2d_ok_2[1]] == id[l_2d_ok_2[2]]]
# False
If you want to generate a multidimensional list, you can nest list comprehensions.
l_3d = [[[0] * 2 for i in range[3]] for j in range[4]]
print[l_3d]
# [[[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]]]
l_3d[0][0][0] = 100
print[l_3d]
# [[[100, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]]]
For tuples and arrays
You can initialize tuples as well as lists.
Note that a tuple with one element requires ,.
- A tuple with one element requires a comma in Python
t = [0,] * 5
print[t]
# [0, 0, 0, 0, 0]
For array type, you can pass the initialized list to the constructor.
- array — Efficient arrays of numeric values — Python 3.9.0 documentation
import array
a = array.array['i', [0] * 5]
print[a]
# array['i', [0, 0, 0, 0, 0]]